Patterned photoresist layers are commonly used as masks in plasma-assisted etching processes, such as reactive ion etching (RIE). Plasma-assisted etching processes generally expose the photoresist to elevated temperatures which often causes out-gassing of trapped nitrogen from the photoresist and flowing of the photoresist material. For photoresists which have thicknesses of more than 12 .mu.m, gas bubbles, blisters, and large changes in the patterned dimensions often occur, which detrimentally affect the ability of the photoresist to act as an effective etch mask. The detrimental effects often limit the smallest dimension that can be reliably reproduced in these etch processes.
One prior, art approach to solving these problems has been to use metal as an etch mask, which is more stable since it does not contain nitrogen. The metal is deposited over the layer to be etched, and is then patterned by wet etching using a patterned photoresist mask. After the metal has been pattern etched, the photoresist is removed. A major drawback of this approach is that the pattern resolution in the metal mask cannot be any better than the resolution of the photoresist mask, and is often worse due to the isotropic characteristic of the wet etching process used to etch the metal mask.
These resolution problems can be addressed by patterning the metal layer with a metal lift-off process, whereby a photoresist layer is formed and patterned prior to depositing the metal layer. This photoresist layer is thicker than the metal layer, and has a pattern which is the negative, or compliment, of the desired pattern of the metal layer. The metal layer is then formed over the photoresist mask such that the metal layer has vertical discontinuities at the perimeters of the photoresist patterns. The discontinuities expose small portions of the photoresist so that a stripper can reach the photoresist and dissolve it. When the photoresist is exposed to the stripper and dissolves, the unwanted metal which forms the negative image of the desired metal pattern is lifted off and removed from the substrate. While this approach has improved resolution, it nonetheless entails additional steps over using a single photoresist mask, which increases the costs of manufacturing. Additionally, the metal layer has to be stripped after being used as a mask, which increases costs and may create compatibility problems between the metal etchant and the underlying substrate layers.
Accordingly, there is a need for an etch mask for plasma-assisted etching processes and other high-temperature processes which have high resolution capability, which do not substantially degrade under high-temperature processing conditions, and which do not substantially increase overall processing complexity.